Why You Can Get Pregnant on Birth Control

Medically Reviewed by Traci C. Johnson, MD on July 12, 2020

Almost half of the over 6 million pregnancies in the United States every year are (whoops!) accidents. A lot of the time, it’s because people skip birth control or don’t use it like they’re supposed to.

But about 5% of the time, women using reliable birth control find themselves unexpectedly pregnant. This is because while all of these methods have a “perfect use” rate, the “typical use” rate is much lower. Day-to-day life can be so hectic that it’s easy to forget to pop a pill, replace a patch on schedule, or even get a birth control shot.

Here’s a look at the effectiveness of the most common types of birth control and how you can lower your chances of a surprise pregnancy.

The Birth Control Pill

Oral contraceptives contain hormones that stop ovulation. If you use them perfectly, they’re a great way to prevent pregnancy, with a 99.7% effectiveness rate. But in reality, a lot of women forget to take them every day, so the typical use rate is only 91%. Other reasons why your pill might not work include:

If you have any of these issues, talk to your doctor about using condoms as a backup method, or switch to another form of birth control entirely.

If you skip a dose, take it as soon as you can. If you’ve missed more than two pills, take them as soon as you remember, and continue to take pills daily while you use a backup method of birth control like condoms for the next week. If you’re on the mini-pill or the progestin-only pill, take it within the same 3 hours every day (For example, if you normally take one at 7 a.m. and then take it after 10 a.m. one day, you’re more likely to get pregnant).

Patches and Rings

These work similarly to the birth control pill. You either insert a ring into your vagina (AnnoveraNuvaRing) or place a patch on your belly, upper arm, buttocks, or back (Ortho Evra). Like the pill, they’re over 99% effective when you take them exactly like you should. But if you don’t, they’re only 91% effective. They can fail because:

  • You don’t put a new one in on time every month, or it’s out of your vagina for more than 2 days during the weeks you need to wear it. The patch can’t do its job if it falls off or if you don’t put a new one on at the right time every week.
  • You take certain meds. The same drugs and supplements that make the pill less reliable also affect the ring and patch the same way.
  • You’re carrying extra weight. Research has found that the patch doesn’t work as well for women who weigh over 200 pounds.

If you use one of these methods, and it falls off or you forget to change it in time, reinsert or replace it within 48 hours. (If it’s been more than 2 days, use a backup method for the next 7 days.)

Birth Control Shot

You get Depo-Provera, or the birth control shot, every 3 months. It contains the hormone progestin, which stops ovulation. With perfect use, it’s 99.8% effective. Sometimes people forget to get an injection on time.

It’s important to get your follow-up shots within 10 to 15 weeks after your last one. If you wait longer than that, you’ll need to use a backup method of birth control for a week.

Birth Control Implant

This is a thin, matchstick-size rod that your doctor inserts into your arm. It releases hormones that prevent pregnancy for up to 3 years. It’s virtually foolproof, since it’s already in your body and you don’t have to remember to take it or to use it the right way. As a result, both the typical and perfect use is over 99.9%.

If you’ve had your implant for 3 years and you still don’t want to get pregnant, you have to replace it. Otherwise you could end up with an unexpected little bundle of joy.


IUDS -- which your doctor inserts into your vagina -- prevent sperm from reaching an egg. They’re more than 99.2% effective whether you use it perfectly or not. Unlike other forms of birth control, you can’t forget to take it or use it the wrong way. Once it’s in, you’re protected from pregnancy for anywhere from 3 to 10 years. There are two types of IUDs: copper (ParaGard) and hormonal (Kyleena, Liletta, Mirena, and Skyla).

If you do get pregnant, it’s usually because the IUD has slipped partly or completely out of the uterus.

If this happens, see your doctor immediately. There’s a higher risk of life-threatening complications such as ectopic pregnancy, in which the egg implants outside of the uterus. You’ll need to get checked for this. Even if the pregnancy is in the right place (the uterus), the doctor will still need to try to remove the IUD as there can be an increased risk of miscarriage if it’s left in place.

Vaginal Contraception

Vaginal contraception is inserted into the vagina prior to sex to create an inhospitable environment for sperm so it won’t reach the egg for fertilization. These come in several forms including foam, jelly, tablet, cream, suppository, or dissolvable film. 

Normally, during sex, the vagina’s PH level rises to allow sperm to move towards the reproductive canal. Spermicides contain chemicals which kills the sperm before it makes its journey. A new prescription non-hormonal gel called Phexxi keeps the PH level of vagina at its normally acidic level, causing the sperm to die without progressing.

Each of these methods has to be used prior to sex and given time to have the desired effect within the vagina (usually 15 minutes). If used properly, spermicides are 70-80% effective but work better when you combine it with a condom or diaphragm. Phexxi is considered 86% effective when used correctly.

Barrier Methods

Barrier methods of birth control like the diaphragm, cervical cap, or male or female condom physically block sperm from entering your uterus. They’re much less reliable than hormonal methods of birth control -- the male condom, for example, is 98% effective with perfect use, but only 82% otherwise. That means 18 women who regularly use condoms with their partner will end up pregnant within a year. Condoms often break or aren’t put on correctly.

These methods work better if you pair them with spermicide, a type of birth control with chemicals that stop sperm from reach an egg.

Fertility Awareness Methods (FAMs)

Also called “natural family planning” and the “rhythm method,” these help you track your menstrual cycle so that you know when you’re ovulating. To do this, you take your temperature every day, check your cervical mucus, or chart your cycle on a calendar. If you use at least one of these methods, and you follow it perfectly, there’s less than a 5% chance you’ll get pregnant. But that can be tough for most women, which is why the typical effectiveness rate hovers around 76%.

If you use multiple FAMs together, they’re more likely to work. But this still isn’t a good option for anyone who has irregular menstrual cycles.

Early Signs of Pregnancy

If you miss a period, take a pregnancy test to make sure that you’re not pregnant. Other signs include tiredness, bloating, having to pee a lot, moodiness, nausea, and tender, swollen breasts. Most pregnancy tests will be positive by the time you miss your first period, but if yours isn’t, it’s still important to see your doctor if you’re more than a week or two late to rule out any other health conditions and to confirm that you aren’t pregnant.

Show Sources


Guttmacher Institute: “Unintended Pregnancy in the United States.”

Morbidity and Mortality Weekly Report: "Appendix D: Contraceptive Effectiveness,” “Contraception: Birth Control Methods.”

Contraception: “Obese women need higher or continuous dose for oral contraceptive success,”

American College of Obstetricians and Gynecologists: “Clinical Challenges of Long-Acting Reversible Contraceptive Methods,” “Barrier Methods of Birth Control: Spermicide, Condom, Sponge, Diaphragm, and Cervical Cap,” “Progestin-Only Hormonal Birth Control: Pill and Injection,” “Combined Hormonal Birth Control: Pill, Patch, and Ring,” “Long-Acting Reversible Contraception: Intrauterine Device and Implant,” “Clinical Challenges of Long-Acting Reversible Contraceptive Methods,” “Barrier Methods of Birth Control: Spermicide, Condom, Sponge, Diaphragm, and Cervical Cap.”

University of Michigan Medicine: “Missed or Irregular Periods.”

National Health Service (UK): “Your Contraception Guide: What if I'm on the pill and I'm sick or have diarrhoea?” “Contraceptive Patch,” “Which medicines affect my contraception?” “How soon can I do a pregnancy test?”

The Canadian Journal of Infectious Diseases: “Antibiotic and oral contraceptive drug interactions: Is there a need for concern?”

National Institutes of Health: “Correcting oral contraceptive pharmacokinetic alterations due to obesity. A randomized controlled trial.”

Mayo Clinic: “NuvaRing (Vaginal Ring),” “Symptoms of pregnancy: What happens first.”

Journal of Family Practice: “What hormonal contraception is most effective for obese women?”

Henry J. Kaiser Family Foundation: “Intrauterine Devices (IUDs): Access for Women in the U.S.”

Healthlink BC: “Barrier Methods of Birth Control.”

Knight, J. The Complete Guide to Fertility Awareness, Routledge, 2016.

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